Two contrasting evolutions of quasi-linear convective systems encountering the northeastern U.S. coastal marine environment

Two contrasting evolutions of quasi-linear convective systems

encountering the northeastern U.S. coastal marine environment

Kelly Lombardo and Brian A. Colle

School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY

Though the evolution of quasi-linear convective systems (QLCSs) over central U.S. land areas has been well documented, there is considerably less known about the interaction of QLCSs with land-ocean boundaries. It is unknown why some systems quickly decay when encountering the coast while others maintain their intensity. This becomes particularly important in the coastal regions of the northeastern U.S. (i.e. New York City and Long Island) where millions of people live, since many of these QLCSs can produce damaging winds as well as flash flooding.

To understand the differing processes between decaying and surviving QLCS events, 2 events will be contrasted: the 23 July 2002 decaying event and the 31 May 2002 sustaining event. Using both observations as well as high-resolution Weather Research and Forecasting (WRF) simulations down to 2-km grid spacing with a 500 m nest, we will examine the role of the low-level temperature advection, low-level vertical wind shear, as well as mechanisms forcing ascent along the convective lines.

There are several important differences between these 2 events. Synoptically, the 31 May sustaining QLCS was located 150 km southeast of a surface cold front, under a 925-hPa warm air advection maximum as it crossed the coastline, which is consistent with composite analyses of 9 northeastern U.S. sustaining QLCS events. This low-level advection helped destabilize the atmosphere (MUCAPE 700 J kg^-1), as well as contribute to the development of a 750 m deep inversion over the coastal waters. Strong (15 m s^-1) southwesterly winds associated with this warm air advection in conjunction with a 15 m s^-1 southerly 500 m offshore jet contributed to 0-2.5 km vertical wind shear values of 15 m s^-1.

Conversely, the decaying 23 July QLCS was collocated with a surface cold front as it encountered the Atlantic coastline, with minimal temperature advection associated with the convective line. MUCAPE values during the 23 July event ranged from 1200-1600 J kg^-1 along the coast and offshore, indicating greater ambient instability than during the 31 May event. In the absence of warm air advection, the shallow marine was more shallow, ~300 m deep, with low-level shear values half that of the 31 May event (7.5 m s^-1). Composite analyses of 32 decaying QLCS events yielded analogous synoptic features as well as instability values.

Preliminary analysis shows that as the decaying 23 July QLCS moved offshore, it remained collocated with the surface front, with the leading edge forced by a surface-based density current. Though RKW reasoning cannot be applied exclusively given that the QLCS was partially synoptically forced, the low-level shear may have been to weak to balance the vorticity of the cold pool. While the cold pool magnitude was comparable during the 31 May QLCS event, the low level shear was twice as large, implying that the greater shear helped support the QLCS. Furthermore, as the sustaining QLCS moved offshore, forcing along the leading edge transitioned from a surface-based density current to an elevated, bore-like feature. We hypothesize that the deeper marine inversion may have helped support the transition to this elevated forcing mechanism. Further analysis is required to diagnose the role of the low-level wind shear during this elevated phase.